Diffraction Activities
Gratings and Lasers
Introduction: When a monochromatic light source like a laser is shined
through a grating, it produces a series of bright spots on a screen. Simple
measurements and geometry can be used to determine the slit spacing of the
grating or the wavelength of the light.
Directions: Unless you have access to multiple green lasers and laser safety
goggles for the whole class, this is best done as a demonstration lab. Set up a
green laser about 2 meters from your white board. Put a grating in front
oriented so that the diffraction pattern is horizontal and can be seen by the
class. Mark the locations of the laser spot and the first order maxima on both
sides. Now turn the laser off and have students measure the perpendicular
distance from the grating to the board (A) and the distance from the laser
spot to the first order maxima (O) on both sides. Average the 2 O
measurements together. Calculate the slit spacing of the grating using your
measurements, the geometry of the experiment setup, the wavelength listed
on the green laser (usually about 532 nm), and the equations for multiple slit
diffraction. Repeat for a red laser and determine the wavelength using the
slit spacing found in the first exercise.
Sin() = wavelength/d
 = tan-1 (O/A)
where d is the grating slit spacing
Measuring CD Pit Spacing
Introduction: A CD consists of a reflective surface with pits “burned” into it.
As the laser scans the CD, the pit prevents its reflection which is read as a 0,
a reflection is a 1. This allows data to be stored and read in digital form. The
pits form a spiral that is read from the inside of the CD outward. The spacing
from one spiral is uniform throughout the CD and can be determined
because the CD acts like a grating. If a red laser is used it is safe and cheap
enough for students to do this activity.
Reflective Spot
Pit
1 0 1 0 1 0 1 0 1 0
Close-up of a CD
Directions: Open the CD case and put the CD so that the label is facing
inward. The case then acts as a stand. Shine the laser so it has a 45o incident
angle to the CD. If the laser hits the CD even with the center hole it will
produce a horizontal diffraction pattern. Place a white sheet vertically so that
it is parallel to the laser and catches the main reflection and first order
maximum of the diffraction pattern. Measure the distance from the CD to the
sheet (A) and the distance between the main reflection and the first order
diffraction (O). The equations to calculate d are the same as the multiple slit
diffraction equations used in the grating activity above. See the below
diagram for the geometry of the setup. You also can do this activity with a
DVD. The closer pit spacing reveals why DVDs store more data.
Measuring Hair Thickness
Introduction: Diffraction and interference effects are used to make precise
measurements of very small distances, such as the pit spacing of a CD.
Another interesting application of this is measuring the thickness of a hair.
Tape a hair across a hole that is cut in a piece of cardboard so that it is
taught. Place the hair so it is in front of the laser. The hair acts as a double
slit. The geometry and equations are the same as the grating activity above.
The only difference is that you will need to have the laser beam travel the
length of the room to obtain a good interference pattern. The pattern will not
be as well defined, measure from the center of the pattern to the center of the
first bright spot. Create a data table fro the whole class and compare the
different hair thicknesses.